1. Field
The present disclosure relates to the use of photonic links for signal distribution in antenna systems. More particularly, the present disclosure relates to a reconfigurable, bipolar RF-transversal filter that may be implemented via the application of wavelength division multiplexed (WDM) and optical-MEMS photonic technologies.
2. Description of Related Art
To discuss the closest prior art, some technical background for the synthesis of finite impulse response (FIR) transversal filters will be provided.
In general, photonic transversal filters may enable one to accomplish an assortment of signal processing operations. These signal processing operations may include, but are not limited to, coarse pre-filtering or fine frequency channelization for RF-signals that have been modulated onto optical carriers. Modulation of RF signals onto optical carriers may be used in photonically remoted antenna systems, such as the antenna system 100 illustrated in FIG. 1.
In the photonically remoted antenna system 100 depicted in FIG. 1, analog optical fiber links are used to transmit the received RF signal (from the antenna element 101 at the front end) to a base-station at the back-end of the system 100, where additional signal processing may be performed in a typically more secure and benign environment. In this manner, the RF or signal-processing hardware elements at the antenna system front end are minimized. In the system 100 depicted in FIG. 1, the RF-signal (received by the antenna 101) is modulated onto one or more optical carriers from continuous wave lasers 103 via the use of an optical modulator 102 (e.g. an electro-optic modulator (EOM) or electro-absorption modulator (EAM)). A first photonic transversal filter 104 may be used to accomplish some selective pre-filtering of the modulated RF-signal. Using a second EOM 106 and an optical local oscillator (LO) 105 signal, the optical carrier modulated with the RF signal can be downconverted to a convenient intermediate-frequency (IF) band signal. This IF signal can then be channelized by a channelizer 107 implemented by a WDM transversal filter. Conversion of the IF optical signal to an electrical signal may be accomplished by a photodetector 108. The photonic pre-processing and channelization steps typically allow for the reduction of the noise bandwidth seen by the analog-to-digital converters (ADC's) 109 used after photodetection. Channelization and pre-processing also serve to alleviate the load of digital signal processors 109 installed in such antenna systems 100.
As indicated above, a photonic transversal filter may be implemented from a FIR filter. FIG. 2A shows the general schematic of a unipolar FIR filter, realized using photonic tapped delay-lines. As shown in FIG. 2A, an optical signal is coupled into a series of optical splitters 201, which split off a portion of the optical signal, as denoted by the tap coefficients a1 to a5. The optical splitters 201 are separated by optical delay elements 202. In FIG. 2A, τd denotes the cascaded time-delay steps. The portions of the optical signal split or tapped from the optical splitters 201 are summed by a summing element 203, which then may provide an output to a photodiode 204 for conversion of the optical signal to an electrical signal.
For an optical input that consists of an impulse (δ-function), one obtains an impulse response h(t) that is given by:
      h    ⁡          (      t      )        =            ∑      n      N        ⁢                  a        n            ⁢              δ        ⁡                  (                      t            -                          n              ⁢                                                          ⁢                              τ                d                                              )                    where an denotes the nth tap-coefficient. FIG. 2B shows the impulse response for the five tap filter shown in FIG. 2A.
The Fourier transform of this impulse response h(t) gives the following frequency response for the filter:
      F    ⁡          (      ω      )        =            ∑      n      N        ⁢                  a        n            ⁢              ⅇ                              -            j                    ⁢                                          ⁢          n          ⁢                                          ⁢          ω          ⁢                                          ⁢                      τ            d                              
The RF-response of the filter is given by |F(ω)|2. In the above, it is assumed that the delay increment, τd, is much longer than the optical coherence time of the optical carrier. Thus, the signals in the different delay paths combine incoherently, i.e., as a sum of optical intensities. Therefore, the an's represent intensity weighting coefficients. As such, they are restricted to values given by positive real numbers, if only unipolar detection is used, as depicted in FIG. 2B.
To synthesize a given filter response F(ω), one needs to find the appropriate tap-coefficients an. The free spectral range (FSR) of the filter response thus synthesized (i.e. from unipolar taps) is given by FSR=(fc)1=τd−1 and its finesse F is approximately N, the total number of taps. For taps of equal weight, i.e., equal an's, the side lobe suppression ratio (SLSR) is only 13.5 dB. To achieve a higher SLSR, one needs to have filter architectures that can apodize the tap coefficients an. Finally, the single polarity FIR filter depicted in FIG. 2A will always exhibit a low pass response centered at DC—a bandpass feature that is undesirable for frequency channelization applications. Bipolar taps allow for the removal of the low pass response centered at DC.
FIG. 3A illustrates a filter architecture where tap-delays of opposite polarities are interlaced in the time-domain. As shown in FIG. 3A, an optical signal is first coupled into an upper branch 310 and a lower branch 320 by an optical coupler 301. Each branch 310, 320 comprises a series of optical splitters 201, which split off a portion of the optical signal in each branch 310, 320, as denoted by the tap coefficients c1 to c10. The optical splitters 201 are separated by optical delay elements 202. The portions of the optical signal split or tapped from the optical splitters 201 from each branch 310, 320 are summed by the summing elements 203.
In the FIR filter shown in FIG. 3A, the impulse responses of the upper and lower branches 310, 320 are directed, respectively, to the positive and negative inputs of a double-balanced receiver 330 (differential photodetector pair). The use of such a differential detector pair 330 allows an, the tap weight coefficients for the impulse response h(t), to be positive or negative real numbers. Notice that an extra time-delay 202 equal to τd/2 is added in the lower branch 320 of the filter. Specifically, this extra delay-loop time-shifts, by τd/2, all the impulse responses sent to the negative input port of the double-balanced receiver 330. After photodetection, an overall impulse response that is interlaced in a bipolar fashion is obtained—with tap-spacings equal to τd/2, as can be seen in FIG. 3B. FIG. 3C shows the frequency domain response of the filter depicted in FIG. 3A. From FIG. 3C it can be seen that the lowpass response (centered at DC) is eliminated for this bipolar FIR filter. In fact, as shown in FIG. 3C, its first passband (fc)1=τd−1, and its FSR is precisely 2(fc)1.
FIG. 4 shows a schematic of prior art of an apparatus 400 that may be used to realize bipolar FIR filters. See, for example, N. You and R. A. Minasian, “Synthesis of WDM Grating-based Optical Microwave Filter with Arbitrary Impulse Response,” Digest of International Topical Meeting on Microwave Photonics, paper F-9.2, pp. 223-226. See, also, Fei Zeng, Jianping Yao, and Stephen J. Mihailov, “Fiber Bragg-grating-based All-optical Microwave Filter Synthesis Using Genetic Algorithm,” Optical Engineering, vol. 42, no. 8, August 2003, pp. 2250-2256.
As shown in FIG. 4, an optical input consists of multiple laser wavelengths (λ1, λ2, . . . λM) from one or more lasers 409. A photonic modulator 401 is used to modulate an RF signal onto the optical input having multiple wavelengths. Similar to the filter depicted in FIG. 3A, the modulated optical signal is split between an upper branch 410 and a lower branch 420. Each branch 410 comprises one or more in-line optical attenuator 402 and delay element 403 pairs, a circulator 406, and fiber gratings 407. Note that FIG. 4 depicts two in-line optical attenuator 402 and delay element 403 pairs in each branch 410, 420, but other embodiments may have additional in-line optical attenuator 402 and delay element 403 pairs. Typically, the optical signal in each branch will be split equally among the in-line optical attenuator 402 and delay element 403 pairs.
The tap-coefficients an of this filter architecture are controlled via the use of in-line optical attenuators 402. The optical signal having M wavelengths is first routed through fiber-delays equal to τ0, 2τ0, . . . or Nτ0. The optical signal is then sent, via the circulator 406, to the fiber-gratings 407 that are “stitched” together to generate wavelength-dependent time-delays Δt0, 2Δt0, . . . MΔt0. Line 440 depicts the path that the optical signal takes through the circulator 406 to the fiber gratings 407 and back. The circulators 406 then direct the optical signals to a double-balanced receiver 430 to produce an electrical signal proportional to the difference between the optical signal from the upper branch 410 and the optical signal from the lower branch 420.
Notice that two distinct fiber-gratings 407 are needed to generate, respectively, the wavelength-dependent time-delays for the positive and negative branches. Therefore, the two fiber-gratings 407 must be precisely matched in the time-delay spacings Δt0—for two different sets of incident wavelengths [(λ1, λ2, λ3) and (λ4, λ5, λ6)] in some instances—to properly form the interlaced bipolar response as illustrated in FIGS. 3B and 3C. Otherwise, timing errors between “taps” with different polarities will be generated.
In the novel filter architecture disclosed in this application, only one fiber-grating is used to generate the wavelength-dependent time-delays. Therefore, the novel approach disclosed in this application offers substantial implementation advantages towards the realization of a bipolar filter. In addition, the use of wavelength-independent taps τ0, 2τ0, . . . Nτ0 in the prior art (as illustrated in FIG.4) may place severe constraints on the type of multi-wavelength source one can employ to realize the filter. Typically, the coherence time τc (inverse of the linewidth) of the optical source used in the apparatus depicted in FIG. 4 must be smaller than τ0 to avoid optical interference effects that stem from the presence of coherent optical fields in the different delay branches (placed before the fiber gratings). Finally, the filter architecture shown in FIG. 4 will not allow for the arbitrary routing of a given time-delay “tap” (e.g. that for the time-delay k1τ0+k2Δt0, where k1<N and k2<M) to the positive (or negative) input port of choice in the differential detector pair.